Genome Evolution through Transposition
Transposons, the so-called "jumping genes," can create heritable variations to their host's genome. These variations can ultimately affect the evolution of their host species. Transposons are classified into two major types: Type I is the retrotransposons that replicates and inserts itself via an RNA intermediate (a "copy and paste" mechanism). These transposons usually self-encode their own reverse transcriptase. Type II transposons are sequences of DNA that can be cut and re-inserted elsewhere by transposase enzymes (a "cut and paste" mechanism. Transposons are widely spread across many organisms genomes. In fact, 45-50% of the human genome is believed to be a transposable element. In eukaryotes, Type II DNA transposons can create variation by altering gene function through their insertion or deletion and by inducing chromosomal rearrangement. While Type II transposons are vastly outnumbered by Type I transposons in most eukaryotic genomes, Type II transposons have a tendency to insert themselves into gene-rich portions of the genome, making their effects more pronounced. Why DNA transposons target gene-rich areas is unknown. Insertion and Deletion The insertion and deletion of a transposon within a genome can create new alleles that drive evolution. The insertion of a transposon sequence within a coding sequence obviously has profound effects on the gene product. If the transposon remains as an exon, it will produce an amino acid sequence that may render the product more or less functional than before, thus affecting the fitness of the host. Conversely, the excision of a transposon from a gene-rich neighborhood is often imperfect and leaves behind small deletions, inversions, and stretches of random filler DNA. These slight errors can have similar mutagenic effects that may provide a survival advantage to the host. For instance, Tol2, a common DNA transposon present in many species, is present in the Medaka fish. When the Tol2 transposon is inserted within the promoter sequence of a pigmentation gene, the Medaka fish will exhibit an albino phenotype. When Tol2 is excised perfectly, the wild type coloration is expressed. But when Tol2 is imperfectly excised, as it would be in vivo, new coloration patterns (new alleles) are seen. Interestingly, these new coloration patterns have been shown to make the fish more attractive to mates. Chromosomal Rearrangements DNA transposons move via a cut and paste mechanism, facilitated by transposase enzymes. If two transposons, or the same transposon in two places, for a double stranded break simultaneously within the same chromosome, that chromosome is liable to undergo an inversion, duplication, or, if close to another chromosome with a similar transposon insertion site, translocation. In Drosophila buzatii, ''the Galileo transposon is responsible for a chromosomal inversion that is strongly linked to other speciation events. Two Galileo transposons are cut simultaneously and the section of the chromosome rotates and reinserts, forming the inversion. Additionally, new chimeric genes are created by the event, leading to further speciation. References Fedoroff NV. November 2012. "Transposable elements, epigenetics, and genome evolution." ''Science. '' Feschotte C, Pritham EJ. 2007. "DNA transposons and the evolution of eukaryotic genomes." ''Annu Rev Genet. Delprat A, Negre B. 2009. "Transposon Galileo generates natural chromosome inversion by ectopic recombination." ''PLoS One. ''